A. Strittmatter

Highly indistinguishable photons from deterministic quantum-dot microlenses utilizing three-dimensional in situ electron-beam lithography

The success of advanced quantum communication relies crucially on non-classical light sources emitting single indistinguishable photons at high flux rates and purity. We report on deterministically fabricated microlenses with single quantum dots inside which fulfil these requirements in a flexible and robust quantum device approach. In our concept we combine cathodoluminescence spectroscopy with advanced in situ three-dimensional electron-beam lithography at cryogenic temperatures to pattern monolithic microlenses precisely aligned to pre-selected single quantum dots above a distributed Bragg reflector. We demonstrate that the resulting deterministic quantum-dot microlenses enhance the photon-extraction efficiency to (23±3)%. Furthermore we prove that such microlenses assure close to pure emission of triggered single photons with a high degree of photon indistinguishability up to (80±7)% at saturation. As a unique feature, both single-photon purity and photon indistinguishability are preserved at high excitation power and pulsed excitation, even above saturation of the quantum emitter.The prospect of realizing building blocks for long-distance quantum communication is a major driving force for the development of advanced nanophotonic devices. Significant progress has been achieved in this field with respect to the fabrication of efficient quantum-dot-based single-photon sources. More recently, even spin-photon entanglement and quantum teleportation have been demonstrated in semiconductor systems. These results are considered as crucial steps towards the realization of a quantum repeater. The related work has almost exclusively been performed on self-assembled quantum dots (QDs) and random device technology. At this point it is clear that further progress in this field towards real applications will rely crucially on deterministic device technologies which will, for instance, enable the processing of bright quantum light sources with pre-defined emission energy. Here we report on enhanced photon-extraction efficiency from monolithically integrated microlenses which are coupled deterministically to single QDs. The microlenses with diameters down to 800 nm were aligned to single QDs by in-situ electron-beam lithography using a low-temperature cathodoluminescence setup. This deterministic device technology allowed us to obtain an enhancement of photon extraction efficiency for QDs integrated into microlenses as compared to QDs in unstructured surfaces. The excellent optical quality of the structures is demonstrated by cathodoluminescence and micro-photoluminescence spectroscopy. A Hong-Ou-Mandel experiment states the emission of single indistinguishable photons.

Maskless epitaxial lateral overgrowth of GaN layers on structured Si(111) substrates

GaN layers are laterally overgrown by metalorganic chemical vapor deposition on structured Si(111) substrates in a single growth process. The substrates are structured with parallel grooves along the Si 〈1–10〉 or perpendicular to the Si 〈1–10〉 direction by standard photolithography and subsequent dry etching. Due to the anisotropic chemical dry etch process, the remaining Si ridges are underetched. The GaN layer grows nearly exclusively on the bottom of the grooves and on the top of the ridges between the grooves. These two growth fronts are completely separated from each other. As a consequence, the GaN layer growing from the ridge area between grooves can extend over the grooves. This process is similar to the so called pendeo-epitaxy process, but is completely mask free during growth and does not require any growth interruption. The improvement of the crystalline and the optical quality of the GaN layer is demonstrated by atomic force microscopy and cathodoluminescence spectroscopy.

Multi-excitonic complexes in single InGaN quantum dots

Cathodoluminescence spectra employing a shadow mask technique of InGaN layers grown by metalorganic chemical vapor deposition on Si(111) substrates are reported. Sharp lines originating from InGaN quantum dots are observed. Temperature dependent measurements reveal thermally induced carrier redistribution between the quantum dots. Spectral diffusion is observed and was used as a tool to correlate up to three lines that originate from the same quantum dot. Variation of excitation density leads to identification of exciton and biexciton. Binding and anti-binding complexes are discovered.

Low-pressure metal organic chemical vapor deposition of GaN on silicon(111) substrates using an AlAs nucleation layer

We have investigated the growth of GaN on silicon by low-pressure metal organic chemical vapor deposition. Good quality GaN layers are grown on silicon(111) using an AlAs nucleation layer. AlAs is thermally stable even at 1050 °C and, unlike GaN and AlN buffer layers, the formation of SiNx on the Si surface is prevented. Single crystalline GaN films are obtained by introducing a thin low-temperature GaN buffer layer grown on the AlAs nucleation layer. The GaN layers are characterized by x-ray diffraction, atomic force microscopy, secondary ion mass spectroscopy, photoluminescence, and cathodoluminescence.

High Quality GaN Layers Grown by Metalorganic Chemical Vapor Deposition on Si(111) Substrates

GaN layers are grown onto silicon (111) substrates by metalorganic chemical vapor phase deposition (MOCVD). The X-ray and photoluminescence spectra as well as the surface morphologies of the layers are comparable to the characteristics of GaN layers grown on sapphire substrates. Linewidths of 610 arcsec in the case of the GaN(0002) reflection in the X-ray ω-scan and 13 meV at 10 K for the dominant excitonic photoluminescence at 3.44 eV as well as a surface roughness below 2 nm (rms) are observed. The high quality has been achieved by a careful optimization of AlAs/AlN buffer layers on the Si substrates.

Recombination dynamics of localized excitons in InGaN quantum dots

Indium-rich fluctuations in ultrathin InGaN layers act at low temperatures as a dense ensemble of quantum dots (QD). This leads to a complex potential landscape with localization sites of widely varying depth for excitons. We report on investigations of the recombination mechanisms of excitons localized in InGaN∕GaN QD structures by time-resolved and spatially resolved photoluminescence (PL) measurements. The structures were grown by metal-organic chemical-vapor deposition on Si (111) substrates. Sharp lines originating from single QDs could be observed. Their PL decays show monoexponential behavior. Similar transition energies have different time constants. Thus, the well-known nonexponential PL decay of the QD ensemble is assigned to the summation of monoexponential decays originating from individual QDs with different exciton lifetimes.

High-power semiconductor disk laser based on InAs∕GaAs submonolayer quantum dots

An optically pumped semiconductor disk laser using submonolayer quantum dots (SML QDs) as gain medium is demonstrated. High-power operation is achieved with stacked InAs∕GaAs SML QDs grown by metal-organic vapor-phase epitaxy. Each SML-QD layer is formed from tenfold alternate depositions of nominally 0.5 ML InAs and 2.3 ML GaAs. Resonant periodic gain from a 13-fold nonuniform stack design of SML QDs allows to produce 1.4W cw at 1034nm. The disk laser demonstrates the promising potential of SML-QD structures combining properties of QD and quantum-well gain media for high-power applications.

Indium redistribution in an InGaN quantum well induced by electron-beam irradiation in a transmission electron microscope

The change of the morphology and indium distribution in an In0.12Ga0.88N quantum well embedded in GaN was investigated depending on the duration of electron-beam irradiation in a transmission electron microscope. Strain-state analysis based on high-resolution lattice-fringe images was used to determine quantitatively the local and average indium concentration of the InGaN quantum well. In-rich clusters were found already in the first image taken after 20 s of irradiation. The indium concentration in the clusters tends to increase with prolonged irradiation time. In contrast, the locally averaged indium concentration and the quantum-well width do not change within the first minute.

In situ electron-beam lithography of deterministic single-quantum-dot mesa-structures using low-temperature cathodoluminescence spectroscopy

We report on the deterministic fabrication of sub-μm mesa-structures containing single quantum dots (QDs) by in situ electron-beam lithography. The fabrication method is based on a two-step lithography process: After detecting the position and spectral features of single InGaAs QDs by cathodoluminescence (CL) spectroscopy, circular sub-μm mesa-structures are defined by high-resolution electron-beam lithography and subsequent etching. Micro-photoluminescence spectroscopy demonstrates the high optical quality of the single-QD mesa-structures with emission linewidths below 15 μeV and g(2)(0) = 0.04. Our lithography method has an alignment precision better than 100 nm which paves the way for a fully deterministic device technology using in situ CL lithography.

Polarized emission lines from A- and B-type excitonic complexes in single InGaN/GaN quantum dots

Cathodoluminescence measurements on single InGaN/GaN quantum dots (QDs) are reported. Complex spectra with up to five emission lines per QD are observed. The lines are polarized along the orthogonal crystal directions [112¯0] and [1¯100]. Realistic eight-band k⋅p electronic structure calculations show that the polarization of the lines can be explained by excitonic recombinations involving hole states which are formed either by the A or the B valence band.